Catalytic cracking is a petroleum refining process that is applied commercially on a very large scale. A majority of the refinery gasoline blending pool in the United States is produced by this process, with almost all being produced using the fluid catalytic cracking (FCC) process. In the FCC process, heavy hydrocarbon fractions are converted into lighter products by reactions taking place at high temperatures in the presence of a catalyst, with the majority of the conversion or cracking occurring in the gas phase. The FCC hydrocarbon feedstock (feedstock) is thereby converted into gasoline and other liquid cracking products as well as lighter gaseous cracking products of four or fewer carbon atoms per molecule. These products, liquid and gas, consist of saturated and unsaturated hydrocarbons.
In FCC processes, feedstock is injected into the riser section of a FCC reactor, where the feedstock is cracked into lighter, more valuable products upon contacting hot catalyst circulated to the riser-reactor from a catalyst regenerator. As the endothermic cracking reactions take place, carbon is deposited onto the catalyst. This carbon, known as coke, reduces the activity of the catalyst and the catalyst must be regenerated to revive its activity. The catalyst and hydrocarbon vapors are carried up the riser to the disengagement section of the FCC reactor, where they are separated. Subsequently, the catalyst flows into a stripping section, where the hydrocarbon vapors entrained with the catalyst are stripped by steam injection. Following removal of occluded hydrocarbons from the spent cracking catalyst, the stripped catalyst flows through a spent catalyst standpipe and into a catalyst regenerator.
Typically, catalyst is regenerated by introducing air into the regenerator and burning off the coke to restore catalyst activity. These coke combustion reactions are highly exothermic and as a result, heat the catalyst. The hot, reactivated catalyst flows through the regenerated catalyst standpipe back to the riser to complete the catalyst cycle. The coke combustion exhaust gas stream rises to the top of the regenerator and leaves the regenerator through the regenerator flue. The exhaust gas generally contains nitrogen oxides (NOx), sulfur oxides (SOx), carbon monoxide (CO), oxygen (O2), ammonia, nitrogen and carbon dioxide (CO2).
The three characteristic steps of the FCC process that the cracking catalyst undergoes can therefore be distinguished: 1) a cracking step in which feedstock is converted into lighter products, 2) a stripping step to remove hydrocarbons adsorbed on the catalyst, and 3) a regeneration step to burn off coke deposited on the catalyst. The regenerated catalyst is then reused in the cracking step.
A major breakthrough in FCC catalysts came in the early 1960's, with the introduction of molecular sieves or zeolites. These materials were incorporated into the matrix of amorphous and/or amorphous/kaolin materials constituting the FCC catalysts of that time. These new zeolitic catalysts, containing a crystalline aluminosilicate zeolite in an amorphous or amorphous/kaolin matrix of silica, alumina, silica-alumina, kaolin, clay or the like were at least 1,000-10,000 times more active for cracking hydrocarbons than the earlier amorphous or amorphous/kaolin containing silica-alumina catalysts. This introduction of zeolitic cracking catalysts revolutionized the fluid catalytic cracking process. New processes were developed to handle these high activities, such as riser cracking, shortened contact times, new regeneration processes, new improved zeolitic catalyst developments, and the like.
The new catalyst developments revolved around the development of various zeolites such as synthetic types X and Y and naturally occurring faujasites; increased thermal-steam (hydrothermal) stability of zeolites through the inclusion of rare earth ions or ammonium ions via ion-exchange techniques; and the development of more attrition resistant matrices for supporting the zeolites. The zeolitic catalyst developments gave the petroleum industry the capability of greatly increasing throughput of feedstock with increased conversion and selectivity while employing the same units without expansion and without requiring new unit construction.
After the introduction of zeolite containing catalysts the petroleum industry began to suffer from a lack of crude availability as to quantity and quality accompanied by increasing demand for gasoline with increasing octane values. The world crude supply picture changed dramatically in the late 1960's and early 1970's. From a surplus of light-sweet crudes the supply situation changed to a tighter supply with an ever increasing amount of heavier crudes with higher sulfur contents. These heavier and high sulfur crudes presented processing problems to the petroleum refiner in that these heavier crudes invariably also contained much higher metals with accompanying significantly increased asphaltic content.
The effects of metals such as Ni—V—Na present in the feedstock and/or deposited on the catalyst during processing of the feedstock have been described in the literature as to their highly unfavorable effect in lowering catalyst activity and selectivity for gasoline production and their equally harmful effect on catalyst life. In particular, vanadium, at high concentrations in the feed, is especially detrimental to catalyst life. Accordingly, the presence of metal contaminants in the feedstock presents a serious problem during FCC processing. Common metal contaminants are iron (Fe), nickel (Ni), sodium (Na), and vanadium (V). Some of these metals may promote dehydrogenation reactions during the cracking sequence and result in increased amounts of coke and light gases at the expense of gasoline production. Some of these metals may also have a detrimental effect on the cracking catalyst stability and crystallinity. During the cracking catalyst regeneration process, metals present in the catalyst itself may volatilize under the hydrothermal conditions and re-deposit on the catalyst. Silicon (Si) is an example of such a metal. All of these metals, whether initially present in the feedstock, the cracking catalyst, or some other compound present in the FCC reactor, may lead to loss of activity, selectivity, stability, and crystallinity of the active component of the cracking catalyst.
As previously noted, vanadium poisons the cracking catalyst and reduces its activity. The literature in this field has reported that vanadium compounds present in feedstock become incorporated in the coke which is deposited on the cracking catalyst and is then oxidized to vanadium pentoxide in the regenerator as the coke is burned off. One possible pathway by which vanadium reduces catalyst activity involves vanadium pentoxide reacting with water vapor present in the regenerator to form vanadic acid. Vanadic acid may then react with the zeolite catalyst, destroying its crystallinity and reducing its activity.
Because compounds containing vanadium and other metals cannot, in general, be readily removed from the cracking unit as volatile compounds, the usual approach has been to passivate these compounds under conditions encountered during the cracking process. Passivation may involve incorporating additives into the cracking catalyst or adding separate additive particles along with the cracking catalyst. These additives combine with the metals and therefore act as “traps” or “sinks” so that the active component of the cracking catalyst is protected. The metal contaminants are removed along with the catalyst withdrawn from the system during its normal operation and fresh metal trap is added with makeup catalyst so as to effect a continuous withdrawal of the detrimental metal contaminants during operation. Depending upon the level of the harmful metals in the feedstock, the quantity of additive may be varied relative to the makeup catalyst in order to achieve the desired degree of metals passivation.
Patents including disclosure of the use of alkaline earth compounds, including magnesium oxide, to mitigate the effects of vanadium include U.S. Pat. No. 4,465,779, U.S. Pat. No. 4,549,548; U.S. Pat. No. 4,944,865; U.S. Pat. No. 5,300,469; U.S. Pat. No. 7,361,264; WO 82/00105; GB 218314A; EP-A-020151 and EP-A-0189267. In some of these references, the magnesium oxide is contained in discrete particles, separate from the particles of zeolite cracking catalyst.
Efforts to develop products and processing modifications to mitigate vanadium passivation are by no means limited to the use of alkaline earth material. Certain perovskites such as barium titanate are employed commercially. Perovskites are expensive. Perovskites are not considered to be very effective in reducing SOX emissions in regenerator flue gas. Alkaline earth material, especially magnesium oxide, offers the additional benefit of reducing SOx in regenerator flue gas from cracking units. See, for example, WO 82/00105 (supra).
There is strong motivation to utilize the inherent vanadium binding and SOx capturing capacity of magnesium oxide in FCC operations utilizing feedstocks having a high content of vanadium. References cited above give some indication of past efforts to produce magnesium oxide based vanadium passivating particles adapted for co-circulating with zeolite cracking catalysts. Commercial success has not measured up to the motivation. One primary challenge was to provide a metals passivator in a physical form of particles sufficiently attrition-resistant for use in FCC, while maintaining the magnesium in most reactive form (oxide). Magnesium oxide without a binder/matrix is unsuitable for use in an FCC unit when it must be circulated through the reactor and regenerator of an FCC unit along with cracking catalyst particles. This is because particles of magnesium oxide readily break down into a powder when subjected to attritive forces. Note that in one of the earliest proposals to use magnesia in an FCC unit to combat SOx (U.S. Pat. No. 3,699,037), the material was circulated in the regenerator to bind SOx. The magnesia attrited during such use, eventually to be withdrawn from the regenerator with flue gas without circulating in the cracker, as would be required to achieve vanadium passivation. Because of the friable nature of magnesium oxide particles, the material did not circulate with the catalyst during the FCC cycle.
Numerous patents, including several of those cited above, disclose formulations based on composites of magnesia with kaolin clay. Kaolin clay is a widely used matrix component for cracking catalyst because it is inexpensive and has potential binding properties. Also, it is relatively catalytically inert in calcined form and is a prime candidate as a matrix/diluent for a vanadium passivator based on magnesia, wherein catalytic activity is not desired. An advantage of using kaolin clay as a matrix/diluent is that it can readily be formed into substantially catalytically inert particles by forming a dispersed concentrated fluid slurry feed for spray drying, to form microspheres in a rotary atomizer or nozzle, spray dryer. When dried, especially when calcined at a temperature above the kaolin dehydroxylation temperatures, the calcined kaolin also serves as a binding function.
Several of the references noted above provide examples of MgO/kaolin microspheres prepared by means including spray drying, but they do not disclose the composition of the feed slurry to the spray dryer. They do not provide information about attrition-resistance. There is no indication that the inventors were concerned with attrition-resistance or steam stability of the products. In the case of WO 82/00105, the matrix was a mixture of kaolin and silica-alumina gel, a conventional matrix for zeolite crystals in an active cracking catalyst. Silica-alumina is a material known to possess catalytic activity.
Those skilled in the art of handling kaolin are aware that introduction of magnesium ions into kaolin slurries causes the slurry to flocculate and thicken. This has been used with benefit in the formulation of various clay-based drilling mud. However, flocculation and thickening, especially in short time periods, causes formidable problems in producing magnesia/kaolin products useful for FCC wherein particles of appreciable magnesium oxide content are produced in spray dryers. Slurry instability results in variable particle packing resulting in particle size distribution and particle void volume. Void volume and packing variability is a major root cause in attrition control issues. It is a simple matter to provide a dispersed kaolin slurry that is sufficiently fluid at a high enough concentration (e.g., 50% solids) to produce coherent microspheres. However, if kaolin is spray dried at low solids, e.g., 10%, the microspheres will fall apart before they can be hardened by calcination. If magnesium is added to such a high solids fluid dispersed slurry of kaolin in more than a trace amount, the slurry will flocculate and thicken. If enough magnesium ions are introduced, a solid gel forms and the slurry cannot be formed into microspheres by spray drying using known technology. Addition of magnesium oxide to a kaolin slurry in amount sufficient to produce spray dried particles having a sufficiently high MgO content for effective vanadium passivation will result in a slurry that cannot be spray dried in continuous commercial spray drying equipment. This problem has plagued the pursuit of developing attrition-resistant spray dried microspheres containing magnesia with a clay diluent which meet the criteria for a good vanadium trap: attrition-resistance; high capacity for vanadium trapping; good vanadium passivation; and very high trapping efficiency (i.e., fast vanadium uptake).
To produce such particles it is necessary to overcome the difficulty caused by flocculation of a dispersed slurry of kaolin by the incorporation magnesium ions, resulting in thickening or even gelation of the slurry and, ultimately, the inability to formulate a slurry of sufficiently high solids content to produce attrition-resistant spray dried microspheres. The need to control flocculation and thickening to achieve hardness was counter-balanced by the need to produce microspheres that were sufficiently porous to function as an effective magnesium passivator.
Commonly assigned U.S. Pat. No. 5,300,469, mentioned above, discloses vanadium passivation particles in the form of spray dried attrition-resistant microspheres composed of a minor amount of magnesium oxide, a major amount of calcined kaolin clay admixed therewith and a small amount, relative to the magnesium of oxide, of an in situ formed amorphous magnesium silicate cement. The particles have a low surface area and have minimal cracking activity. Products of the disclosed invention are obtained by processing steps comprising mixing magnesium oxide or magnesium hydroxide, hydrous (uncalcined) kaolin clay and sodium silicate with water to form a dispersed fluid slurry which is aged. During aging, the basic magnesium compound reacts with sodium silicate to produce magnesium silicate which functions as a binder in the finished product. Optionally colloidal silica, additional kaolin clay or both are added to the aged slurry. The proportion of magnesium oxide relative to sodium silicate is controlled to assure that only a small amount of magnesium silicate is formed, leaving the bulk of the magnesium oxide unreacted and available in the product to passivate metals and, if desired, to bind SOx. The slurry is spray dried to form microspheres which are calcined under conditions sufficiently mild to dehydrate the kaolin while preventing or minimizing the formation of crystalline magnesium silicate and/or magnesium aluminates.
One key manufacturing parameter which the inventors of U.S. Pat. No. 5,300,469 believe is crucial is the avoidance of large amounts of magnesium silicate formation in such a way that only marginal reaction between the two components can take place. Another key parameter is a moderate calcination temperature which avoids the formation of significant amounts of crystalline magnesium silicate and aluminate, thereby preserving the bulk of the MgO throughout the process. A significant amount of crystalline magnesium silicate or aluminate would be a quantity such that more than about half of the magnesium oxide is consumed in forming that crystalline material. Still another key parameter was assuring that the magnesium oxide was well dispersed from the time it was mixed with other ingredients to form a slurry until spray drying. Thus, in order to produce these microspheres in a form sufficiently attrition-resistant for use in FCC involved careful selection of starting materials (MgO, clay and binder) and making a careful selection of slurry preparation procedures prior to spray drying. The patentees stated that magnesium should not be used in the form of magnesium hydroxide or magnesium carbonate, as both give rise to problems due to particle shrinkage, ultimately resulting in vanadium traps having poor attrition resistance.
Despite the best efforts to form an effective magnesium oxide-containing metal trapping particle, which is also attrition resistant and can be readily processed and formed on a commercial scale, problems still persist. Thus, the presence of silicates whether from the kaolin or additional binders, result in reactions with the magnesium oxide to form magnesium silicate, which is not an effective metals trap in refinery applications as magnesium oxide. Moreover, the poor stability of magnesium oxide slurries is still problematic. When aged, such slurries often thicken and gel in 24-48 hours rendering scheduling of production difficult and variable. Also, when the slurry viscosity changes, controlling the final product particle size is challenging which necessitates modifications which add to the cost of the process. Further, magnesium oxide slurries have relatively low solids content, about 20-22% by weight. The low solids content reduces the spray dryer rate and, thus, the productivity of the plant, raises energy consumption and, thus, total product cost. Further, due to the low solids penalty, magnesium oxide levels in the final product may not be as high as desired for effective metals trapping.